As the demand for oil and gas resources becomes higher increasingly in the world, more and more deep and ultra-deep wells, and shale gas well are drilled. Thus, the performance of drilling fluid treating agents must meet more demanding requirements. Wherein, the problem of wellbore stability in the well drilling process has always been a challenge not solved completely yet in the world, and accidents such as borehole caving, shrinkage, pipe sticking, and bit balling, etc. resulted from wellbore instability cause severe economic loss every year in the petroleum industry.
90% wellbore instability accidents in the drilling process happen in the shale formation, where the dispersion and exfoliation of shale resulted from hydrated swelling of clay in the shale is the root chemical factor for wellbore instability. As shale gas extraction becomes an important strategy in China, it is urgent task for drilling fluid researchers to ensure that the shale formation remains stable in the drilling process.
In the development in the past few decades, oil based drilling fluids always were an ideal choice for solving the problem of hydrated swelling of shale. However, as the national environmental laws become strict increasingly, the application of oil based drilling fluids is restricted owing to the severe damages of oil based drilling fluids to the environment. Hence, water-based drilling fluids with strong shale inhibition property have become a hot spot in the research again.
The inhibition property of a drilling fluid usually depends on the performance of the shale inhibitor in the drilling fluid. At present, in almost all shale inhibitors, the inhibition mechanism is realized by weakening the hydrated swelling of clay.
However, in existing commonly used shale inhibitors, the hydrated swelling of clay can't be inhibited completely. In addition, the pressure transfer from the drilling fluid to deep formation through nano-to-micro-sized pores and micro-fractures in shale is also a major cause for wellbore instability. At present, there is no appropriate drilling fluid plugging material that can effectively plug nano-to-micro-sized pores and micro-fractures.
Therefore, it is urgent task to seek for a technique that can be used to stabilize wellbore effectively.